1. Field of the Invention
The present invention is in the field of noninvasive cardiac monitoring, and, more particularly, in the field of color Doppler blood flow imaging wherein the velocity of blood flow in a vessel or through an orifice is represented by color images on a display with different colors representing different velocities and different directions.
2. Description of the Related Art
Color flow mapping using the Doppler effect is a method for noninvasively imaging blood flow through the heart and other vessels by displaying flow data on a twodimensional echocardiographic image such as an image on a color CRT monitor, or the like.
Color flow mapping operates by directing an ultrasonic signal toward the heart from an ultrasonic transducer positioned outside the patient and using a sensing transducer to monitor the return echo generated by the signal. As is well known in the field of echocardiography, the return signal will arrive at a sensing transducer at a time that is determined by the distance that the signal has travelled from the generating transducer to a sound reflecting material and back to the sensing transducer. The signal will have an intensity that is determined by the structural characteristics of the reflecting material. These characteristics of the sensed echo have been used in conventional echocardiography to map the physical structure of the heart and other related structures of the patient. This technique uses color encoding to display velocities from multiple sample volumes using multi-gated pulse Doppler techniques.
Color Doppler echocardiography further monitors the frequency of the sensed echo to determine additional information. It has been known that an echo from a moving object is shifted in frequency with respect to a transmitted signal in accordance with the velocity of the moving object and the direction in which the object is moving with respect to the sensing transducer. This so-called Doppler effect has been used, for example, in radar, sonar, and the like, to monitor moving objects and determine their velocities. In color Doppler echocardiography, the Doppler effect is used to determine blood flow characteristics, and, in particular, is used to identify constrictions in vessels, regurgitation (i.e., leaks or back flow) in heart valves, leaks between the chambers of the heart, and the like.
The operation of color Doppler echocardiography is well understood in the medical field and is described, for example, in Joseph Kisslo, et al., "DOPPLER COLOR FLOW IMAGING," Churchill Livingston, Inc., New York, 1988 (ISBN 0-443-08563-3), and in "COLOR ATLAS of Real-Time Two-Dimensional Doppler Echocardiography," Second Edition, Ryozo Omoto, M. D., Editor, Shindan-to-Chiryo Co., Ltd., Tokyo, 1987 (ISBN 0-8121-1116-8). Additional background information regarding color Doppler echocardiography can be found in the two references.
A number of color Doppler imaging systems are commercially available for performing color Doppler echocardiography. One such system is the Model EUB-151 Ultrasound Sector Scanner with Digital Scan Converter available from Hitachi Medical Corporation of Tokyo, Japan, available in the United States as the Model CVC-151 from Biosound. Basically, such systems operate by using a probe having an ultrasonic transducer that generates an ultrasonic signal as a series of pulses and a sensing transducer to sense the return echoes. The probe is positioned to direct the ultrasonic signal pulses toward a region of interest in the patient (e.g., toward the heart). The probe includes circuitry to cause the ultrasonic signal pulses to be scanned in a fan-like pattern so that the ultrasonic signal pulses pass through a planar area of the region of interest. By measuring the time of arrival of the return pulses from the region of interest, a two-dimensional representation of the structural characteristics of the region of interest can be generated. In addition, as discussed above, the return signal pulses are shifted in frequency when they are reflected from moving material in the region of interest. The color Doppler imaging system includes a processor and other suitable electronic hardware and software to measure the frequency shifts of the return signal pulses and to correlate the frequency shifts with the times of arrival of the return signal pulses to generate a two dimensional representation of the flow in the region of interest. The two-dimensional representation is displayed in color on a display monitor (e.g., a color CRT) with different colors representing different velocities and directions of movement in the region of interest. Such movement is generally the flow of blood through the heart and other vessels.
In a typical color Doppler imaging system, two distinct ranges of colors are used to indicate flow direction and velocity. For example, in one exemplary system, a range of blue colors from light blue to dark blue are used to indicate flow toward the probe and a range of red colors are used to indicate flow away from the probe. Other colors may of course be used, and some imaging systems allow the operator to select the colors used to represent flow velocities and directions and to select the range of velocities represented by a particular range of colors.
It has been found that in color Doppler imaging systems using ultrasonic pulses, such as described above, a phenomenon referred to as "aliasing" occurs. Since the pulsed operation of a color Doppler imaging system is basically operating in a sampling mode, the aliasing phenomenon occurs because of the inability of the system to faithfully record velocities, as well as the direction of flow, above a certain velocity (i.e., one-half the Nyquist limit) for a given depth setting ("range") and ultrasound transducer frequency. When aliasing occurs, the color displayed on the display monitor will appear as the color associated with a high velocity in the opposite direction. For example, in an imaging system where bright blue represents the maximum velocity toward the probe and bright red represents maximum velocity away from the probe, when the blood flow towards the probe exceeds the maximum velocity, the displayed color will switch from bright blue to bright red. Since a skilled operator of the imaging system will know that the blood flow cannot make an abrupt transition in direction at the maximum velocities indicated, the operator will know that the transition from bright blue to bright red on the display is a distinct indication that the blood flow at a particular location is moving at a rate referred to as the aliasing velocity. Since the operator can adjust the ranges of velocities that can be represented by the red and blue color ranges, the operator will know that the blood flow at the red/blue interface has a velocity equal to the maximum velocity of the range.
Heretofore, the color Doppler imaging systems have been used to identify imperfections in the cardiac system as indicated by blood flow between chambers of the heart caused by openings in the chamber walls, regurgitation of blood caused by incomplete closure of a heart valve, disturbed flow caused by obstructions, and the like. Such imperfections are identifiable qualitatively by observing the color patterns on the display monitor. However, quantitative measurement capability has not been perfected to enable a clinician to determine non-invasively the quantity of blood flowing through a leak or through an obstruction in a vessel or valve so that the clinician can determine whether the defect requires immediate surgical intervention or medication and so that the clinician can make comparisons over a period of time to determine whether the defect is becoming worse.
There have been attempts to make quantitative measurements of blood flow distal to an orifice such as an orifice in the chamber wall between two chambers of a patient's heart. Such attempts have generally not been very successful because of a number of machine factors such as the system gain of the imaging system, the transmit power, the frame rate, and the like, which can vary from measurement to measurement, thus preventing accurate quantification of the measured flow.
It has recently been suggested that the red/blue color interface representing blood flow proximal to an orifice can be used to provide quantitative measurements of blood flow in patients having mitral regurgitation. See, for example, G. S. Bargiggia, et al., "QUANTITATIVE ASSESSMENT OF MITRAL REGURGITATION BY COLOR DOPPLER ANALYSIS OF FLOW CONVERGENCE REGION: USEFULNESS OF CONTINUITY EQUATION," Sixth International Congress on Echocardiography, Rome, Italy, June 23-25, 1988, page 140. As discussed above, the red/blue color interface represents the aliasing velocity which corresponds to the limit of the range of the velocity that can be represented by a distinct color in a given direction. Thus, all pixels on the displayed image at the red/blue color interface have the same aliasing velocity. The red/blue color interface is thus referred to as the isovelocity line. Bargiggia, et al., suggest that the flow velocity increases as the flow stream lines converge toward an orifice (i.e., a partially open mitral valve), and that symmetry requires that all velocities be the same at the same radial distance from the orifice. Bargiggia, et al., then suggest that the isovelocity line can be used to model a hemispherical surface at a radial distance r from the orifice wherein the velocities are the same. By multiplying the surface area of the surface by the velocity, Bargiggia, et al., proposed to calculate the flow rate.
Applicants have compared the flow rates calculated using the Bargiggia, et al., method with known flow rates in experiments wherein the actual flow rate can be accurately determined by empirical methods. As a result of the experiments, it has been determined that the Bargiggia, et al., method is not sufficiently accurate to be used for quantitative measurements of blood flow rate. Thus, a need continues to exist for an accurate method of using color Doppler imaging techniques to quantify flow rates through an orifice.